1. Field of the Invention
The present invention relates to a quarter-wavelength coupled line type directional coupler, and in particular, to a floating potential conductor coupled quarter-wavelength coupled transmission line type directional coupler comprising a cut portion formed in a ground plane conductor.
2. Description of the Related Art
Conventionally, directional couplers have been used when constituting a 90-degree combiner or divider. In particular, in a microwave circuit, the directional couplers are applied to various kinds of microwave circuits such as a balanced amplifier or a balanced mixer. FIGS. 17 and 18 show a conventional quarter-wavelength coupled line type directional coupler employing two microstrip lines arranged so as to be electromagnetically coupled with each other.
Referring to FIGS. 17 and 18, a ground plane conductor 12 is formed on a semiconductor substrate 11, and then, a dielectric layer 21 is formed on the ground plane conductor 12. On the dielectric layer 21, two coupled microstrip conductors 31 and 32 are formed as separated apart by a predetermined distance so as to be electromagnetically coupled with each other. In the above-mentioned structure, each of the microstrip conductors 31 and 32 has a length of a quarter wavelength, i.e., (1/4) .lambda.g (where .lambda.g is a guide wavelength) in the longitudinal direction. When analyzing the above-mentioned conventional directional coupler by a quasi-TEM approximation method (See J. Reed, et al. "A method of analysis of symmetrical four-port network" IRE Trans., MTT-4, 1968) according to the even-odd mode excitation method which is known to those skilled in the art, the directional coupler is excited with in-phase in the even mode, while it is excited with out-of-phase excitation in the odd mode. Characteristic impedances Zodd and Zeven respectively in the odd mode and even mode of the respective coupled transmission lines of the directional coupler are expressed by the following equations (1) and (2). ##EQU1## where .epsilon. represents a dielectric constant of the dielectric layer 21, .mu. represents a permeability of the dielectric layer 21, C.sub.1 represents an electrostatic capacity between the microstrip conductors 31 and 32 and the ground plane conductor 12, and C.sub.12 represents an electrostatic capacity between the microstrip conductors 31 and 32.
The coupling factor K between the two microstrip lines of the conventional directional coupler can be expressed with the above-mentioned characteristic impedances Zodd and Zeven by the following equation (3). ##EQU2##
However, since the coupling factor K expressed by the equation (3) can not be further increased in the conventional directional coupler, it is difficult to obtain specifications of the structure for achieving equal power dividing and power combining. Therefore, such directional couplers have not been often used conventionally in apparatuses which include a monolithic microwave integrated circuit (referred to as an MMIC hereinafter).
For the above-mentioned reasons, a hybrid ring employing a transmission line such as a microstrip line or the like has been widely used upon constructing a microwave circuit. However, the hybrid ring requires a large circuit area, and this results in that the microwave circuit to be implemented becomes relatively large.
In order to overcome the above-mentioned drawbacks, there has been tried to perform a method for decreasing the circuit area thereof by laminating metal conductors and thin film dielectric layers on a semiconductor substrate with a multi-layer structure and by using the resulting product as a microstrip line. However, due to use of the thin film electric insulating layer, the width of the conductor of the resulting microstrip line becomes narrow. In the case of a 90-degree hybrid ring, it is necessary to provide a transmission line having a line length of one guide wavelength .lambda.g of the frequency to be used, and therefore, the insertion loss of the transmission line increases. In other words, there have been such a drawback that neither desired power distribution nor desired synthetic or combining characteristics cannot be obtained and such a problem that the loss is increased in the MMIC employing such a hybrid ring.